Major histocompatibility complex (MHC) class I molecules present intracellular pathogen-derived peptides to T-cells, which become activated upon engagement of clonotypic ocp T-cell antigen receptors (TCRs) and of costimulatory receptors. Distant homologs of MHC class I molecules have altered functions and interact with different receptors and T-cell subsets. Among these are human MICA and MICB, which are cell stress-inducible surface molecules that are not associated with antigenic peptide or nonpeptide ligands. They have a limited expression in intestinal epithelium and epithelial tumors, can be potently induced in virus-infected cells, and are recognized as self-antigens by a subset of T-cells with 78 TCRs that is frequently enriched in epithelial sites. In addition, MICA/B function as ligands for an activating receptor, NKG2D, which is broadly expressed on lymphocyte subsets. Engagement of NKG2D can trigger natural killer (NK) cells and augment responses by antigen-specific CD8+ a(3T-cells and y8 T- cells. The here proposed research aims at advancing knowledge of this immunological system that may be capable of mobilizing diverse effector cells against tumor and virus-infected cells. Studies planned under Specific Aim I will define the molecular mechanisms underlying the stress-inducible expression of the MICA/B genes. Transcriptional induction will be measured, relevant transcription factors identified, and their binding to predictable sites within a defined promoter context determined. Studies under Specific Aim II are designed to demonstrate interactionsof MICA/B with y5 TCRs and to investigate how variable y and 8 chains contribute to TCR specificity. Further goals are to quantify the subset of 78 T-cells specific for MICA/B and to determine the role of NKG2D in activation. Studies under Specific Aim III will investigate the induction of MICA/B in cells infected by cytomegalovirus and the function of NKG2D in promoting virus-specific CD8+ a[3 T-cell responses. This and additional experimental systems have been chosen to demonstrate that NKG2D can costimulate TCR-mediated T-cell activation.